It is estimated that, in the FCC (i.e., Fluid Catalytic Cracking) unit of a refinery, the energy taken away by the flue gas from the regenerator occupies about 26% of the energy consumption of the whole unit. At present, in order to reduce a waste of energy in the flue gas, a flue gas turbine is generally employed to recycle energy in the flue gas. Besides, in order to ensure a long-period stable operation of the flue gas turbine, it is generally required that the concentration of fine particles in the inlet gas of the flue gas turbine should be lowered below 200 mg/m3; it also should be guaranteed that particles greater than 10 μm are totally eliminated. However, the outlet flue gas of the regenerator are dust-laden high-temperature gases, in which the catalyst concentration (under standard state, the same below) is in general from 300 to 1200 mg/m3, and an average particle size thereof is 15 to 30 μm, which cannot meet the demands for the long-period stable operation of the flue gas turbine. In order to ensure the long-period stable operation of the FCC unit, for energy saving and cost reducing, and for reducing pollutions to the atmosphere caused by dust particles contained in the flue gas, a third-stage cyclone separator (hereafter referred to as a third-stage cyclone) is generally provided in a flue gas energy recovery system of the FCC unit.
At present, the third-stage cyclone system usually employs the multi-tubular cyclone separators scheme to purify the dust-laden flue gas. The so called multi-tubular cyclone separator is formed by mounting a plurality of cyclone tubes with a small diameter (the diameter is usually smaller than 300 mm) in parallel in a pressure bearing housing. According to the mounting way of the cyclone tubes, the third-stage cyclone can be divided into the vertical type multi-tubular third-stage cyclone and the horizontal type multi-tubular third-stage cyclone. For those small FCC units, the vertical type third-stage cyclones are usually employed, in comprehensive consideration of factors such as the manufacturing construction, the application effect and the investment economy. For example, Chinese patents CN 2568308Y and CN 201304370Y both relate to the vertical type multi-tubular third-stage cyclones. As for the large FCC units, on the other hand, the horizontal type third-stage cyclones are generally employed because the number of single tubes therein is relatively large, the diameter of the vertical type third-stage cyclone is too large consequentially, and the partition board is too thick and easily deformed. For example, Chinese patents CN 2275907Y, CN 2526075Y and CN 201132137Y all relate to horizontal type multi-tubular third-stage cyclones.
However, the multi-tubular third-stage cyclones have some problems in applications. With the increase of residual blending ratio of raw material and the regeneration temperature, the operations of the FCC unit become very unstable, which brings about the following problems of the third-stage cyclone system.
Firstly, in order to achieve a high gas-solid separating efficiency, the diameter of the cyclone tube is usually designed to be very small; correspondingly, the gas flow rate through a single tube is small. In order to ensure the handling capacity, generally we can only increase the number of the single tubes. In this way, since the number of the single tubes is large, both the floor area and material costs of the third-stage cyclone system increase correspondingly. Besides, the layout structure of the system is complex, which brings about inconvenience for mounting and repair.
Secondly, due to excessive pursuits for separation efficiency of the single tube, the inlet gas velocity of the single tube of the third-stage cyclone is too high, causing a severe wear of the single tube. When the inlet gas velocity of the single tube of the third-stage cyclone is too high, the smashing of catalyst particles will also be aggravated, which will increase the fine powders escaping from the third-stage cyclone, forming severe deposition near the dust exhaust opening of the single pipe of the third-stage cyclone, thereby affecting the operation of the flue gas turbine.
Thirdly, since the inlet gas velocity of the single tube is too high, a pressure drop of the third-stage cyclone reaches up to 15-20 kPa, thus the pressure drop and energy consumption of the system are increased.
The BSX type third-stage cyclone (patents CN 201006498Y and CN 201205524Y) developed by the China Petrochemical Corporation employs a plurality of individual PV type cyclone separators having a large diameter, which are hanged inside the third-stage cyclone housing; the diameter thereof is 800-1200(1300) mm. Compared to the multi-tubular third-stage cyclone, the BSX type third-stage cyclone removes the double-layer partition board of a large thickness and the small diameter single pipes requiring high manufacturing accuracies, instead, it employs several simple structured large diameter cyclone separators, which reduces difficulty in manufacturing, construction, installation, facilitates the repair and replacement of the third-stage cyclone. Besides, since the problems of deformation of the partition board and short-circuit flow and back-mixing of the single tube are not exist, the overall efficiency of the equipment will not decrease after the plurality of cyclones are combined, thus the reliability is high, and the adaptability is good. However, a separation efficiency of the large diameter individual cyclone separator in the BSX type third-stage cyclone is lower than that of the single pipe in the multi-tubular third-stage cyclone. Furthermore, in order to achieve high separation efficiency, the inlet gas velocity of the BSX type third-stage cyclone reaches above 32 m/s. Therefore, the third-stage cyclone system not only has a high pressure drop, but also has an obvious vibration phenomenon in operation, which will cause severe equipment wear and catalyst smashing.
Currently, a dust concentration at the gas outlet of the third-stage cyclone is usually about 150 mg/m3, and the corresponding pressure drop of the third-stage cyclone system ranges from 15 kPa to 20 kPa. As such, the overall separation efficiency of the third-stage cyclone system is not ideal because the pressure drop is high.
In order to improve the separation efficiency and reduce the pressure drop, it is doable to couple a moving bed in the separation system. The moving bed refers to a gas-solid two-phase flow system, which is intermediate regime between a fixed bed and a fluidized bed, and mainly includes three forms, namely, counter-current flow, co-current flow and cross flow. Since the moving bed has advantages of resisting high temperature, simple structured, free of rotating components, long running period and being operable circularly and continuously, it is widely employed in technological processes such as particle drying, filtration and catalytic reforming and the like. Since the granular bed has a good capability in filtering and capturing fine particles and can be adapted to high-temperature high-pressure conditions, in the past twenty years, the moving bed has also been used in the field of gas purification.
For example, Chinese patent (CN 2042374U) has proposed a filter for granules with an auto-moving bed, of which the granular bed applies a louver structure to realize a cyclic and continuous operation of the bed granules. The dipleg at the bottom of the equipment employs a U-shaped pipe delivery structure, and then the delivery of the granules is realized by using a pulsing air flow method.
The counter-current flow type moving bed filter with particle layers (CN 1552503A and CN 1552504A) developed by the Thermal Power Research Institute is used for removing dusts from coal gases or fumes under high temperature and high pressure. The contact of coal gases and fumes with the counter-current flows of the granules can realize high separation efficiency. However, due to the existing problem of local fluidization phenomenon of the bed granules and the dust particles, the equipment is limited to the small gas flow rate, the low operational flexibility, the uneven distribution of granules and gases, as well as the high pressure drop.
The Shanxi Institute of Coal Chemistry of Chinese Academy of Sciences couples a cross flow type granular bed with a surface filter plate (CN 1236660A) for improving the capability of the cross-flow type granular bed in capturing micron-grade particles. It applies the flow of descending granules to restrain the formation of a filter cake on the surface of the filter plate, so as to achieve a stable operating pressure drop. However, due to the blockage of the filter plate and formation of a filter cake on the surface, the pressure drop of the equipment is high. In addition, the purpose of limiting a filter cake on the surface is achieved because of the scouring effect of the freely descending particles. However, the balancing process is hard to be guaranteed.
The prior art also includes other equipment that couple granular bed with other type separator. However, on the whole, the current equipment does not have an ideal effect in separating dust particles from moving bed granules. It is inevitable that the moving bed granules will be mixed with dust particles, thereby affecting the subsequent sustainable recycling utilization of the granules.